1. Field of the Invention
The invention relates to a cast-rolling plant for the continuous casting of metal strip, especially aluminum strip, which has two counterrotating continuous casting rolls, between which a gap is formed.
2. Description of Related Art
In cast-rolling, a fluid molten metal is cast between two continuous casting rolls that are rotating in opposite directions and are positioned horizontally, vertically or at an angle. In this operation, the strip solidifies between the two continuous casting rolls and is continuously advanced in the process.
So-called two-roller strip casting of aluminum strip is a method that has been applied for the past few years. Using this method, strip thicknesses are produced in the range of 1 mm to 10 mm. The method is characterized by two continuous casting rolls that are usually situated vertically above one another, between which a casting gap is produced that corresponds to the desired strip thickness.
Continuous casting rolls of the current type of construction have a cylindrical core, usually made of steel, which is used to guide cooling water, and a jacket connected to the core. In the cast-rolling of steel, substances of high heat conductivity, such as copper or copper alloys are usually employed as the material for the jacket. In the casting of nonferrous metals, steel jackets are normally used.
As material for the production of steel jackets, high-strength steels are used having alloy elements C, Mn, Ni, Cr, Mo, V, which, at room temperature, have strengths between 800 MPa and 1,200 MPa. The disadvantage of these materials is their limited heat conductivity, which usually lies in the range of 25 to 50 W/m·K.
Because of the low thermal conductivity of the steel jackets, achievable casting speeds are also limited. These days, depending on the alloy, casting outputs in the range of 0.7 to 1.2 t/m/h are reached. Auxiliary sets of machines of a casting-roll plant, such as melting and casting furnaces as well as coiling equipment are designed for these average casting speeds.
In the case of jackets made of copper or copper alloys, copper materials are predominantly used, having thermal conductivities in the range of 200 to 370 W/m·K. Especially when using special alloys based on copper as well as cobalt and beryllium is it possible to produce aluminum strip using copper continuous casting rolls, under production conditions.
Because the thermal conductivity of the copper alloys is up to ten times greater, considerably greater heat can be removed from the melt, so that substantially greater casting speeds are able to be reached on the cast-rolling plants. Up to this point, in experiments, casting outputs of 2.5 t/m/h to 2.8 t/m/h have been achieved.
Besides high strength and elongation limit (Rp0.2≧450 MPa), copper alloys suitable for continuous casting rolls additionally have to have high values for elongation A5.
A disadvantage when using continuous casting rolls having a copper jacket is the comparatively high cost of the continuous casting rolls, which are only amortized if there are correspondingly high casting outputs, which, however, is not always a given.